1. Field of the Invention
The present invention relates to a Coriolis mass flow meter that retains a measurement tube with a space apart therefrom.
2. Related Art
The Coriolis mass flow meter is a flow meter of a type obtaining a mass flow by measuring a Coriolis force based on the fact that the Coriolis force acting on a mass point of a mass m moving toward or away from a center of rotation of a rotational vibrating system at a velocity V is proportional to a product of the mass m and the velocity V.
Compared with differential, electromagnetic, and volumetric flow meters, the Coriolis mass flow meter has various superior characteristics such as being capable of obtaining amass flow directly, having no mechanically movable portion which causes abrasion, being superior in maintainability, and, in principle, being capable of measuring a density by measuring the number of vibrations of the measurement tube.
For example, JP-A-3-41319 discloses a Coriolis mass flow meter using a U-shaped measurement tube as illustrated in FIG. 10. The measurement tube is composed of a single U-shaped measurement tube 1, and the cantilevered U-shaped measurement tube 1 repeatedly vibrates upward and downward about points fixed via mounting flanges 2a and 2b at a resonant frequency applied thereto.
Fluid to be measured flowing into the measurement tube 1 causes distortion of the measurement tube 1 by a Coriolis force generated by a flowing velocity with respect to the measurement tube 1 when flowing from an inlet port toward a curved portion of a U shape, and causes distortion of the measurement tube 1 in an opposite direction by the Coriolis force when flowing from the curved tube portion toward an outlet port, which causes a vibration of the measurement tube 1.
A vibrator 3 is provided at a distal end of the measurement tube 1, which forms the U shape, displacement detection sensors 4a and 4b are mounted on the measurement tube 1 at both sides of the curved portion.
Fluid to be measured is flowed in the measurement tube 1, and the vibrator 3 is driven, so that the measurement tube 1 is vibrated. A Coriolis force of Fc=−2 mω×ν, where ω is an angular speed of the vibrator 3 in a direction of vibration and ν is a flow velocity of the fluid to be measured, works, and the mass flow may be measured by detecting an amplitude of the vibration which is proportional to the Coriolis force Fc by using the displacement detection sensors 4a and 4b and calculating the detected result.
A metallic tube having a high rigidity is normally used for the measurement tube 1 of the Coriolis mass flow meter of the related art to prevent a measurement error caused by deformation such that the U-shaped curved tube portion bows down under its own weight even when the measurement tube 1 is filled with the fluid to be measured, for example. However, the metallic tube is hard to process and thus it is difficult to obtain the metallic tubes having the same features by processing. When using, the size and the weight of a supporting structure are significantly large, and cost is also high.
As a consequence, usage of a synthetic resin tube as the measurement tube is conceivable. However, usage of such a synthetic resin tube may require a structure suitable for retaining the measurement tube, which is easily deformable specifically at the curved tube portion, and increased in rigidity against a vibration in contrast to advantages in good processability and light weight.